Portable communication devices, such as cellular telephones, personal digital assistants (PDAs), WIFI transceivers, and other communication devices must be capable of communicating using a number of different frequency bands. For example, current portable communication devices may communicate in three, four, or more communication bands.
One of the challenges in designing a portable communication device that operates in multiple bands is providing isolation between the transmit and receive bands. For example, transmit energy in a particular transmit band must not overlap and interfere with a receive band. One way of ensuring this isolation is to implement a surface acoustic wave (SAW) filter at the output of each transmitter. A SAW filter reduces the noise of the transmitter output at the receiver frequency to prevent desensitizing the receiver. However, the SAW filter consumes valuable space on the integrated circuit and is costly to implement. Therefore, with the need for including more and more standards into a single radio frequency (RF) transceiver, eliminating the SAW filter in the transmitter design becomes highly attractive to aid in reducing cost and package size.
In order to eliminate the SAW filter from the transmit path, the noise of the transmitter should be low at the corresponding receive frequency. Unfortunately, it is not possible to reduce the noise of the transmitter to such low levels with available transmitter architectures.
In a modern RF transmitter, upconversion of baseband data is realized by multiplying, also referred to as mixing, the data signal with a carrier signal. Using the trigonometric identity,
            cos      ⁡              (        x        )              ·          cos      ⁡              (        y        )              =            1      2        ⁢          (                        cos          ⁡                      (                          x              -              y                        )                          +                  cos          ⁡                      (                          x              +              y                        )                              )      
The baseband signal is upconverted into two sidebands in the frequency domain. Since only one of the generated sidebands is of interest, the unwanted sideband is rejected by using in-phase (I or sine) and quadrature-phase (Q or cos) signals both for baseband and LO signals.
In an in-phase (I) quadrature-phase (Q) (I/Q) transmitter that relies on a 90 degree phase separation between the in-phase signal and the quadrature-phase signal, one of the most significant circuit elements that limits the achievable performance is the local oscillator (LO) signal generation circuitry in the upconverter. The LO signal can be thought of as a reference signal that is used to upconvert the baseband information signal to a transmit signal. Therefore, phase noise of the LO signal at an offset frequency away from the main frequency directly contributes to the noise of the transmitter output. Specifically, for the offset frequencies which overlap the receive frequency of a different user channel, phase noise performance becomes highly critical. Typically, the circuitry that generates the LO signal occupies a large circuit area on an integrated circuit and consumes high power in order to maintain a low phase noise.
The baseband signal and the LO signal are combined in what is referred to as a “mixer core.” The mixer core is another critical element in the transmit chain. Similar to the circuitry that generates the LO signal, the mixer core also generates noise which directly contributes to noise in the transmitter output. An active mixer typically consumes high power in order to keep noise low. A passive mixer does not consume power, but provides limited isolation between I and Q baseband inputs. Good isolation between the I and Q baseband inputs is vital for most transmitters.
Therefore, it would be desirable to have an upconverter that achieves low phase noise, provides good sideband isolation and that consumes minimal circuit area and power.